<?php 
class ascii
{

function decode($str)
{
    preg_match_all( "/(d{2,5})/", $str,$a);
    $a = $a[0];
     foreach ($a as $dec)
     {
         if ($dec < 128)
         {
            $utf .= chr($dec);
         }
         else if ($dec < 2048)
        {
            $utf .= chr(192 + (($dec - ($dec % 64)) / 64));
            $utf .= chr(128 + ($dec % 64));
         }
         else
         {
            $utf .= chr(224 + (($dec - ($dec % 4096)) / 4096));
            $utf .= chr(128 + ((($dec % 4096) - ($dec % 64)) / 64));
            $utf .= chr(128 + ($dec % 64));
         }
     }
     return $utf;
}

function encode($c)
{
    $len = strlen($c);
    $a = 0;
     while ($a < $len)
     {
        $ud = 0;
         if (ord($c{$a}) >=0 && ord($c{$a})<=127)
         {
            $ud = ord($c{$a});
            $a += 1;
         }
         else if (ord($c{$a}) >=192 && ord($c{$a})<=223)
         {
            $ud = (ord($c{$a})-192)*64 + (ord($c{$a+1})-128);
            $a += 2;
         }
         else if (ord($c{$a}) >=224 && ord($c{$a})<=239)
         {
            $ud = (ord($c{$a})-224)*4096 + (ord($c{$a+1})-128)*64 + (ord($c{$a+2})-128);
            $a += 3;
         }
         else if (ord($c{$a}) >=240 && ord($c{$a})<=247)
         {
            $ud = (ord($c{$a})-240)*262144 + (ord($c{$a+1})-128)*4096 + (ord($c{$a+2})-128)*64 + (ord($c{$a+3})-128);
            $a += 4;
         }
         else if (ord($c{$a}) >=248 && ord($c{$a})<=251)
         {
            $ud = (ord($c{$a})-248)*16777216 + (ord($c{$a+1})-128)*262144 + (ord($c{$a+2})-128)*4096 + (ord($c{$a+3})-128)*64 + (ord($c{$a+4})-128);
            $a += 5;
         }
         else if (ord($c{$a}) >=252 && ord($c{$a})<=253)
         {
            $ud = (ord($c{$a})-252)*1073741824 + (ord($c{$a+1})-128)*16777216 + (ord($c{$a+2})-128)*262144 + (ord($c{$a+3})-128)*4096 + (ord($c{$a+4})-128)*64 + (ord($c{$a+5})-128);
            $a += 6;
         }
         else if (ord($c{$a}) >=254 && ord($c{$a})<=255)
         { //error
            $ud = false;
         }
        $scill .= "&#$ud;";
     }
     return $scill;
}

}
/*

PHP 转 ASCII


require_once "ascii_class.php";
*/
$aa = new ascii;

echo $aa -> encode("a");


?>